Power transmission

ABSTRACT

A pressure regulated variable delivery hydraulic pump includes a control for varying the pump displacement which responds not only to outlet pressure of the pump, but also to the rate of change of pressure and to the rate of change of pump delivery. Regulation is by outlet pressure opposed by two springs in back-to-back relation with a dashpot intermediate the two springs which retards rapid movements of one spring in backing up the other. The dashpot action is further modified by a feedback responsive to the rate of change of pump displacement. This is derived from pressure differentials generated by flow in the displacement varying servomotor system. The result is a servocontrol system which has high gain characteristics for small transients and lower gain for large transients and an increased damping ratio with a reduction in overshoots of pressure transients.

United States Patent [191 Lonnemo Jan. 8, 1974 POWER TRANSMISSION [75]Inventor: Kurt R. Lonnemo, Troy, Mich.

[73] Assignee: Sperry Rand Corporation, Troy,

Mich.

221 Filed: June 13, 1972 [211 App]. No.: 262,397

Primary Examiner-William L. Freeh Assistant Examiner-Gregory P. LaPointeAttorney--Theodore Van Meter [57] ABSTRACT A pressure regulated variabledelivery hydraulic pump includes a control for varying the pumpdisplacement which responds not only to outlet pressure of the pump, butalso to the rate of change of pressure and to the rate of change of pumpdelivery. Regulation is by outlet pressure opposed by two springs inback-toback relation with a dashpot intermediate the two springs whichretards rapid movements of one spring in backing up the other. Thedashpot action is further modified by a feedback responsive to the rateof change of pump displacement. This is derived from pressuredifferentials generated by flow in the displacement varying servomotorsystem. The result is a servocontrol system which has high gaincharacteristics for small transients and lower gain for large transientsand an increased damping ratio with a reduction in overshoots ofpressure transients.

4 Claims, 1 Drawing Figure POWER TRANSMISSION Modern central hydraulicpower systems such as used on aircraft, for example, impose severeperformance requirements upon the hydraulic pump which supplies pressurefluid to the system. It is customary to provide a variable displacementpump with a displacement regulating mechanism intended to maintain aconstant pressure in the fluid delivery line from the pump. Due to thewide and rapid variation in volumetric requirements as one or another ofthe fluid utilizing devices in the system are turned on or off, thepressure transients at the pump are highly fluctuating. It is thefunction of the pump regulator to minimize such transients by quicklyvarying the pump delivery rate to correspond to each instantaneouschange in volumetric demand by the system. Prior pump regulators, ifdesigned for high sensitivity and rapid response, have the drawback ofintroducing overshoots before achieving a steady state after a largetransient. These overshoots can only be avoided by a reduction insensitivity, thus resulting in enlargement of a pressure transientduring its initial phase.

it has been proposed in the copending application of Nicholas F.Pedersen, filed June 13, 1972, Ser. No. 262,398, to improve theperformance of pressure responsive pump regulators by the provision of acompound spring .and dashpot system in the regulator to provide avariable gain characteristic and to introduce a feedback from thedisplacement varying mechanism of the pump which is a first derivativefunction of any pressure transient. This has required the introductionof a special feedback piston and cylinder for pumping fluid into or outof the compound spring and dashpot mechanism.

Where the displacement varying member of the pump is moved by adifferential area servomotor, it is possible, according to the presentinvention, to elimi nate the additional feedback piston and cylinderwhile delivering to the compound spring and dashpot mechanism a firstderivative signal generated solely by flow in the differential areaservomotor system.

These effects are achieved by the provision in a pressure regulatedvariable delivery pump which has an inlet and an outlet with expansiblechamber means for pumping fluid therebetween together with a shiftablemember connected with the expansible chamber means and effective to varythe volumetric pumping capacity, hydraulic regulating means responsiveto outlet pressure for shifting the member, including a differentialarea servomotor; that improvement which comprises a multiple springsystem opposing outlet pressure at the regulating means, including afirst spring acting directly on the regulating means, a second springbacking up the first spring, dashpot means intermediate the two springseffective to retard rapid movements of the second spring in backing upthe first spring, and a fluid connection between the differential areaservomotor.

The drawing is a diagrammatic representation of a variable delivery pumpand pressure responsive control therefore incorporating a preferred formof the present invention. t

In the drawing, the pump is indicated as an axial piston type having aninlet 12 and an outlet 14 with a revolving cylinder barrel 16 driven bya shaft 18. Pistons 20 reciprocate in the cylinder barrel 16 as theyrevolve around on the swashplate 22 which is oscillatable on trunnions24 in the stationary casing 26. The angular position of the swashplate22 is adjusted by means of a differential area servomotor including alarge area piston 28 and a small area piston 30, thus the swashplate 22constitutes a member shiftable to vary the volumetric displacement ofthe pump 10. The foregoing description is merely illustrative of one ofthe many types of variable displacement hydraulic pumps well known tothe art today.

For the purpose of regulating the displacement of the pump 10 tomaintain a substantially constant pressure in the outlet 14, there isprovided a pilot valve assembly 32 which senses small pressure changesin the outlet 14 which occur as the volumetric consumption of fluid bythe system changes and directs fluid toward or away from servopiston 28.Thus, the pilot valve 32 includes a spool 34 which is exposed to outletpressure through a conduit 36. The left hand end of the spool 34 is incontact with a multiple spring system generally designated 38, whichbiases the spool .34 to the right. The spool 34 constitutes a three-wayvalve which, in its neutral position, blocks all flow to or from thepiston 28 through a conduit 40. When shifted to the left, spool 34admits pressure fluid to conduit 40 from conduit 36 and when shifted tothe right, it exhausts fluid from conduit40 to the pump inlet throughconduit 42. A conduit 44 connects servopiston 30 with the outlet. Thus,spool 34 constitutes a pilot valve for controlling the piston 28 inaccordance with changes in pressure in the outlet 14 and, except for themultiple spring and dashpot system hereafter described, is typical ofmany well known pressure responsive regulators for variable displacementpumps.

The multiple spring system 38 includes a first spring 46 which abutsdirectly against the spool 34 and a second spring 48 which serves as aback-up for the spring 46. A screw 50 serves as a means for adjustingthe compression of the two springs 46 and 48 and thus changing theaverage pressure to which the spool 34 responds. Outlet pressure is alsodetermined by hydraulic pressure in the spring chamber set by pilotrelief valve 60. Between the two springs 46 and 48, there is a floatingpiston 52 which slides in the chambers enclosing the two springstodisplace fluid into one chamber and out of the other, or vice versa. Arestricted orifice 54 permits limited communication between the twospring chambers, easily permitting slow movements of the piston andresisting more rapid movements. Thus, the piston 52, its restriction 54, and the two spring chambers constitute a dashpot which tends todampen rapid changes in the position of the left end of spring 46; thatis to say, of the backing up relationship of spring 48 to spring 46.

For the purpose of modifying the dashpot action of the piston 52 andrestriction 54, the rate of change of position of the swashplate 22 isintroduced into the multiple spring system by means of a restrictedconnection 56 from conduit 44 to the left-hand spring chamber. Arestriction 58 in conduit 44 generates pressure changes in its left-handportion which are indicative of the rate of flow in conduit 44 and ofthe speed of motion of swashplate 22. This further modifies or tends toinhibit rapid changes in the position of the left-hand end of spring 46by changing the quiescent flow rate through the orifice 54 causing areaction in opposition to the otherwise normal motions of the piston 52acting as a simple dash-pot. Since the spring chambers are, at steadystate, under outlet pressure, a pilot relief valve 60 can serve to limitmaximum pressure in the system, acting through the servosystem 34, 28,30.

Preferably, the springs 46 and 48 have equal spring rates so that understeady state conditions, they constitute, in effect, a single springhaving half the rate of either spring. Thus, for small or slow motionsof the spool 34, the control system as a whole is extremely sensitive.That is to say, the regulator has a very high gain at or near steadystate conditions. For larger excursions of the spool 34 induced bylarger or more rapid pressure transients, the dashpot action of thepiston 52 and the restriction 54 retard the motion of the left-hand endof spring 46 and thus increase the spring rate which is effective uponthe left-hand end of spool 34. This produces a lowered gain of theservosystem as a whole during more major transients. In effect, thesensitivity of the regulator is high for small transients and is lowerfor larger transients.

In addition, the feedback conduit 56 further modifies the back-up actionof spring 48 upon spring 46 by introducing pressure changes in theleft-hand spring chamber which are opposed to motions of the spool 34.These pressure changes are greatest when the velocity of the swashplate22 is the greatest. Thus, a sort of anticipatory signal results whichhas the effect of reducing the subsequent overshoot of any pressuretransient. The feedback through conduit 56 acting upon the dashpotresults in an increase in the damping ratio of the regulator as a whole.The feedback is, of course, a first derivative function of any pressuretransient. The overall result of the variable gain and the derivativefeedback is a pressure responsive control system which is stable in alarger range of system volume demands than is possible to achieve withconventional controls. This greatly reduces the pressure overshoots andundershoots which are normally associated with flow changes in thehydraulic systems of the type described.

It will be noted that there is under steady state conditions a continualsmall flow from the outlet 14 through 36, 44, 58, 56, 54 and pilot valve60 to inlet. Pressure drops occur at restrictions 56 and 58 resulting ina pressure in the spring chambers (and at the left end of spool 34)which is considerably lower than outlet pressure. This pressure willrise and fall as described during transients in the volumetricrequirements which the fluid utilization part of the system may imposeupon the pump and its regulator.

I claim:

1. In a pressure regulated variable delivery pump having an inlet and anoutlet, expansible chamber means for pumping fluid therebetween, ashiftable member connected with the expansible chamber means andeffective to vary the volumetric pumping capacity, hydraulic regulatingmeans responsive to outlet pressure for shifting the member theregulating means including a differential area servomotor connected toand controlled by a valve responsive pump outlet pressure thatimprovement which comprises a multiple spring system opposing outletpressure at the valve of the regulating means, including a first springacting directly on the valve, a second spring backing up the firstspring, dashpot means intermediate the two springs effective to retardrapid movements of the second spring in backing up the first spring, anda fluid connection between the differential area servomotor and thedashpot for sensing flow rate changes in the servomotor.

2. A pump as defined in claim 1 wherein the dashpot means includes afirst chamber surrounding the first spring, a second chamber surroundingthe second spring and connected to the outlet, a floating pistonseparating the chambers, and a restricted connection between the twochambers.

3. A pump as defined in claim 1 wherein the first and second springshave equal force-displacement rates.

4. A pump as defined in claim 2 wherein a pilot relief valve is providedto regulate the pressure in the first spring chamber and govern the flowthrough the restricted connection between the two spring chambers tothereby vary the pressure differential between the two chambers.

1. In a pressure regulated variable delivery pump having an inlet and anoutlet, expansible chamber means for pumping fluid therebetween, ashiftable member connected with the expansible chamber means andeffective to vary the volumetric pumping capacity, hydraulic regulatingmeans responsive to outlet pressure for shifting the member theregulating means including a differential area servomotor connected toand controlled by a valve responsive pump outlet pressure thatimprovement which comprises a multiple spring system opposing outletpressure at the valve of the regulating means, including a first springacting directly on the valve, a second spring backing up the firstspring, dashpot means intermediate the two springs effective to retardrapid movements of the second spring in backing up the first spring, anda fluid connection between the differential area servomotor and thedashpot for sensing flow rate changes in the servomotor.
 2. A pump asdefined in claim 1 wherein the dashpot means includes a first chambersurrounding the first spring, a second chamber surrounding the secondspring and connected to the outlet, a floating piston sepArating thechambers, and a restricted connection between the two chambers.
 3. Apump as defined in claim 1 wherein the first and second springs haveequal force-displacement rates.
 4. A pump as defined in claim 2 whereina pilot relief valve is provided to regulate the pressure in the firstspring chamber and govern the flow through the restricted connectionbetween the two spring chambers to thereby vary the pressuredifferential between the two chambers.